Lesson 2 Terms

DECNET - Group of communications products (including a protocol suite) developed and supported by Digital Equipment Corporation. DECnet/OSI (also called DECnet Phase V) is the most recent iteration and supports both OSI protocols and proprietary Digital protocols. Phase IV Prime support inherent MAC addresses that allow DECnet nodes to coexist with systems running other protocols that have MAC address restrictions.

SNA - Systems Network Architecture. Large, complex, feature-rich network architecture developed in the 1970s by IBM. Similar in some respects to the OSI reference model, but with a number of differences. SNA is essentially composed of seven layers.

TCP/IP - Transmission Control Protocol/Internet Protocol. Common name for the suite of protocols developed by the U.S. Department of Defense in the 1970s to support the construction of worldwide internetworks. TCP and IP are two best-known protocols in the suite.

Interoperability - Ability of computing equipment manufactured by different vendors to communicate with one another successfully over a network.

OSI Reference Model - Open System Interconnection reference model. Network architectural model developed by ISO and ITUT-T. The model consists of seven layers, each of which specifies particular network functions such as addressing, flow control, error control, encapsulation, and reliable message transfer. The highest layer (the application layer) is closest to the user; the lower layer (the physical layer) is closest to the media technology. The lowest layer is implemented in hardware. The next lowest layer is implemented in hardware and software, while the upper five layers are implemented only in software. The OSI reference model is used universally as a method for teaching and understanding network functionality.

Standards - Set of rules or procedures that are either widely used or officially specified.

Physical Layer - Layer 1 of the OSI reference model. The physical layer defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between end systems.

Data Link Layer - Layer 2 of the OSI reference model. This layer provides reliable transit of data across a physical link. The data link layer is concerned with physical addressing, network topology, line discipline, error notification, ordered delivery of frames, and flow control. The IEEE has divided this layer into two sublayers; the MAC sublayer and the LLC sublayer. Sometimes simply called link layer.

Network Layer - Layer 3 of the OSI reference model. This layer provides connectivity and path selection between two end systems. The network layer is the layer at which routing occurs.

Transport Layer - Layer 4 of the OSI reference model. This layer is responsible for reliable network communication between end nodes. The transport layer provides mechanisms for the establishment, maintenance, and termination of virtual circuits, transport fault detection and recover, and information flow control.

Session Layer - Layer 5 of the OSI reference model. This layer establishes, manages, and terminates sessions between applications and manages data exchange between presentation layer entities.

Presentation Layer - Layer 6 of the OSI reference model. This layer ensures that information sent by the application layer of one system will be readable by the application layer of another. The presentation layer is also concerned with the data structures used by programs and therefore negotiates data transfer syntax for the application layer.

Application Layer - Layer 7 of the OSI reference model. This layer provides services to application processes (such as electronic mail, file transfer, and terminal emulation) that are outside of the OSI model. The application layer identifies and establishes the availability of intended communication partners (and the resources required to connect with them), synchronizes cooperating applications, and establishes agreement on procedures for error recovery and control of data integrity.

Router - Network layer device that uses one or more metrics to determine the optimal path along which network traffic should be forwarded. Routers forward packets from one network to another based on network layer information

Switch - 1. Data link layer device that filters, forwards, and floods frames based on the destination address of each frame. The switch operated at eh data link layer of the OSI model. 2. General term applied to an electronic or mechanical device that allows a connection to be established as necessary and terminated when there is no longer a session to support.

Internet - Term used to refer to the largest global internetwork, connecting tens of thousands of networks worldwide and having a "culture" that focuses on research and standardization based on real-life use. Many leading-edge network technologies come from the Internet community. The Internet evolved in part from ARPANET. At one time, called the DARPA Internet. Not to be confused with the general term Internet.

Data - Logically grouped units of information that move between computer systems.

Introduction
As you learned in Lesson 1, early development of LANs, MANs, and WANs was chaotic in many ways. The early 1980s saw tremendous expansion in the area of network deployment. As companies realized how much money could be saved and how much they could gain in productivity by using network technology, they began adding networks and expanding existing networks almost as rapidly as new network technologies and products were introduced. By the mid-1980s, growing pains from this expansion were being felt. Increasingly, it became difficult for networks using different specifications and implementations to communicate with each other.

What Model was developed to describe networking?
To address the problem, the International Organization for Standards (ISO) researched network schemes like DECNET, SNA, and TCP/IP. As a result of this research, the ISO recognized there was a need to create a network model that would help vendors create networks that would work compatibly and interoperably with the networks. The OSI Reference Model released in 1984, was the descriptive scheme they created. By creating the OSI model, the ISO was providing vendors with a set of standards, thus ensuring greater compatibility and interoperability between the various types of network technologies that were being produced by many companies around the would.

What is the OSI Reference Model?
The OSI Reference Model quickly became the primary model for network communications. Although other models have been created, today, most network vendors relate their network products to the OSI Reference Model when they want to educate users about their products. Thus, the model is the best tool available to people hoping to learn about sending and receiving data on a network.

The OSI Reference Model is not something tangible. Rather it is a conceptual framework specifying the network functions that occur at each layer. In simpler terms, a model is a way of picturing how information travels through networks. Even though all vendors may use the OSI model, there can still be considerable variance among the networks they implement.

What process does the OSI Reference Model describe?
The problem of moving information between computers is divided into seven smaller and more manageable problems in the OSI Reference Model. Each of the seven problem areas is solved by a layer of the model. The seven layers of the OSI Reference Model are: the physical layer, the data link layer, the network layer, the transport layer, the sessions layer, the presentations layer, and the application layer. Because they control the physical delivery of messages over the network, the lower layers of the OSI model are sometimes referred to as the media layers. Because they provide for the accurate delivery of data between computers on the network, the upper layers of the OSI model are sometimes referred to as the host layers.

The OSI Reference Model describes how information or data makes its way from application programs (such as spreadsheets) through a network medium (such as wires) to another application program located on another computer on a network. As the information to be sent descends through the layers of a given system, it looks less and less like human language and more and more like the ones and zeros that a computer understands.

Why is Networking desirable?
As Lesson 1 explained, by networking or connecting computers, printers, routers, switches, and other devices so that they can communicate with each other, it is possible to share information, resources, or get to the Internet.

How does encapsulation allow computers to communicate data?
To understand how networks are structured and how they functions, you should remember that all communications on a network originate at a source and are being send to a destination. The information that is sent on a network is referred to as data or data packets. If one computer (host A) wants to send data to another computer (host B), the data must first be packaged in a process called encapsulation.

As data moves down through the layers of the OSI model, headers are added.